martha raddatz, abc news, washington. >> it's a little hard to believe since tens of thousands of folks have been killed already, use of chemical weapons would be off limits to the syrian regime right now. considering rebels have made advances that may be one of the most powerful weapons the government has in its arsenal right now. again, that's a red line according to the u.s. if chemicals are used, that could draw us into this conflict militarily. that is a whole different level of involvement here, and according to some senior officials, our folks, military have started to run drills in preparation for chemical attacks. that situation growing more serious. >> syria maintaining that they're not preparing for any sort of chemical warfare. though they have begun mixing compounds according to several sources. as she mentioned the sarin gas could be most readily used to fill artillery shells. real quickly, the united nations said monday they're immediately pulling all nonessential employees out of syria because of this new development. >> huge signal there as well. >>> all right. moving o